The cubic structure of the cubane molecule, ${\mathrm{C}}_8{\mathrm{H}}_8$, gives rise to many unusual properties in the solid state. X-ray powder diffraction measurements of solid cubane reveal a large thermal expansion, followed by a strong first-order phase transition at 394 K from an orientationally ordered phase ( $R\overline{3}$) to a noncubic orientationally disordered phase ( $R\overline{3}$ or $R3m$). The resulting volume expansion is $5.4\%$. Model calculations predict the correct low- and high-temperature phases. The orientational potential in the disordered phase resembles a rounded square well over a large angular range. Cubane thus represents an ideal example of a system with large-amplitude collective motions.

• Received 12 February 1997

DOI:https://doi.org/10.1103/PhysRevLett.78.4938